A mobile radio apparatus such as a portable radio receiver, a portable radio transceiver, a portable wireless communication terminal, and a portable telephone set, for example, a cellular phone set, a PHS (Personal Handyphone System) telephone set, or the like is provided with an antenna assembly so as to transmit and/or receive radio signals. A known type of the antenna assembly is retractably attached to a housing of the radio apparatus, typically, a portable one such as the portable telephone set.
An antenna assembly of the type is disclosed in JP-A 3-245603 (Reference I) includes a first antenna portion, a second antenna portion connected to the first antenna portion, and a holder fitted around the second antenna portion so that the second antenna portion is slidable through the holder. The holder is connected to a transceiver circuit in a housing of the portable telephone set. The holder serves to mechanically attach the antenna assembly to the housing of the portable telephone set and serves as an electrical contact for the first and the second antenna portions.
The first antenna portion comprises a helical antenna element, an antenna top formed by plastic molding to surround the helical antenna element for the purpose of protection, and a conductive sleeve having one end connected to the helical antenna element and extending in one direction to be exposed outward from the antenna top.
The second antenna portion comprises a whip antenna element formed by a conductive wire or thin rod and having one end fixed and connected to the sleeve, and an insulator jacket tube covering the whip antenna element to protect the whip antenna element. The second antenna portion has one end coupled to the sleeve and the other end provided with a stopper to inhibit the antenna assembly from dropping off from the holder.
When the antenna assembly is in an extended position, the holder is brought into contact with the stopper to be electrically connected thereto. In this state, the whip antenna element is electrically connected to the transceiver circuit through the stopper and the holder. In this manner, the helical antenna element and the whip antenna element connected in cascade serves to receive a radio signal, because the whip antenna element is electrically connected in cascade to the helical antenna element connected to the one end of the whip antenna element.
On the other hand, when the antenna assembly is in a retracted position, the holder is brought into contact with the sleeve. As a result, the whip antenna element and the helical antenna element are simultaneously connected to the transceiver circuit through the sleeve and the holder and are rendered active. In this event, the whip antenna element retracted in the housing often badly affects circuit components of the transceiver circuit in the housing. In order to avoid the problem, the sleeve and the stopper may be short-circuited in the retracted position by use of, for example, a coaxial structure so that the helical antenna element alone is rendered active while the whip antenna is left inactive, as disclosed in JP-A 5-243829 corresponding to BP-A-2257836 (Reference II).
In the above-mentioned antenna assembly, there is a large difference in the electric characteristic of the antenna, such as a resonance frequency and a VSWR (Voltage Standing Wave Ratio) thereof, between the antenna extended position and the antenna retracted position. In order to solve the problem, a matching circuit is usually used to adjust the difference.
However, the matching circuit must be carefully designed and adjusted in order to obtain excellent characteristics of the antenna assembly both in the antenna extended position and the antenna retracted position. This requires much time and labor.
There is also known in the prior art, for example, U.S. Pat. No. 5,204,687 (Reference III) and JP-B-2646505 (Reference IV) another structure of the antenna assembly where the conductor rod as the whip antenna element is not electrically connected with the helical antenna element but is insulated therefrom. In the structure, the whip antenna element is reliably disabled in the retracted without use of a special support structure such as the coaxial structure as shown in Reference II. While, the whip antenna element only serves for receiving the radio signal in the extended condition because the helical antenna element is no longer connected to the holder.
In detail, Reference IV discloses a dielectric joint member of a generally rod shape which is secured at one end thereof to the top end of the conductor rod of the whip antenna element. The joint member is partially covered with the conductive sleeve and is fitted at the other end portion with a coil bobbin. A helical coil or the helical antenna element is wound on the coil bobbin and is connected to the conductive sleeve. The dielectric cap covers on the coil bobbin, the helical coil element and the top end portion of the conductive sleeve together by, for example, the plastic molding to from the antenna top. In the connection, the conductive sleeve and the topend of the conductive rod of the whip antenna element are fixed to the joint member by plastic molding the joint member in a condition where the conductive sleeve and the top end of the conductive rod are inserted into a mold.
However, the inventor has found out a problem that the antenna assembly disclosed in reference III and IV considerably varies in its transmitting/receiving performance in dependence on the dimension of an axial length of the joint member.
It is an object of this invention to provide an antenna assembly comprising a helical antenna element and a whip antenna element mechanically fixed to each other and electrically separated from each other so that the helical antenna element alone is rendered active in a retracted position and so that the whip antenna element alone is rendered active in an extended position, wherein an excellent characteristic can easily be realized and formation of a matching circuit is also easily performed.
It is a another object of this invention to provide a mobile radio apparatus with the above-mentioned antenna assembly attached to a housing of the apparatus.